Evolution of the North Central Peruvian Subduction Zone: Evidence From Late-Stage Adakite-Like Ignimbrites

The Miocene Yungay and Fortaleza Ignimbrites of the Peruvian Cordillera Blanca are characterised by restricted geochemical variations, with SiO2 >70 wt %, Na2O >4 wt %, Mg numbers from 4 to 30, and strong depletions in Y, Yb and Lu. La/LuN and Sr/Y ratios are high, at >20 and >65 respectively. These geochemical indicators are typical of Phanerozoic adakite-like rocks and Archaean Trondhjemote- Tonalite-Granodiortite (TTG) suites which are often interpreted as partial melts derived from subducted oceanic slab. 40Ar/39Ar dating on Yungay feldspars yields an age range of 4.1 to 7.5 Ma, while K- Ar dating on Fortaleza biotites places them at 4.9 to 5.84 Ma. Both Ignimbrites were erupted during major tectonic modifications to the Peruvian subduction margin, and it is against this complex tectonic backdrop, effectively marking the end of the current 200 Ma Andean Cycle, that the adakite-like Yungay and Fortaleza magmas were generated in rapid succession. There are only two possible source regions for the Peruvian Ignimbrite melts: thick underplated lower crust or the downgoing oceanic slab. Resolving between these allows a full understanding of the magmatic evolution of the Peruvian margin, and carries implications for formation of TTG melts. Radiogenic isotope compositions of the Yungay and Fortaleza Ignimbrites point to a juvenile crustal source (87Sr/86Sr = 0.70547 to 0.70631, 143Nd/144Nd = 0.51245 to 0.51257, ɛ Hf = -3.2 to 0.88, 206Pb/204Pb = 18.767 to 18.783). Direct observation of source mineralogy and melting conditions, through experimental petrology, indicates that pressures of >2.2 GPa and temperatures in excess of 1025° C are needed to generate the chemical compositions of the Ignimbrite melts. Initial melts at source were in equilibrium with a fluid-absent, hydrous source mineralogy consisting of quartz, garnet and clinopyroxene (and possibly zoisite). Quartz-hosted melt inclusions show the Ignimbrite magmas were volatile-rich, and suggest the magmas experienced a short period of crystallisation near 40 km depth after initial generation. The Yungay and Fortaleza Ignimbrites represent the final melts of a short-lived, deep lower crust at ~80 km depth, created by progressive shallowing of the Nazca Plate after break-up of the Farallon Plate at 27 Ma. Adakite-like melts may simply reflect a natural progression in arc geochemistry as a mature subduction system evolves and crust is thickened via tectonic and magmatic events during the final stages of a magmatic cycle.